Dynamic optical spectral filters are a class of filters that can dynamically change the shape of their spectral filter transmission/reflection function. This distinguishes them from single cavity Fabry-Perot tunable filters, for example, in which, while the passband can be tuned across a band of interest, the passband shape itself is static or relatively static.
One proposed application for these dynamic optical spectral filters is as gain flattening filters. These filters are deployed at various stages along the optical fiber communication link to control the relative powers in the channels of some wavelength or frequency band of interest in a wavelength division multiplexed (WDM) optical signal. Gain tilt from optical amplifiers, such as erbium-doped fiber amplifiers (EDFA), or wavelength dependent losses, for example, can be neutralized.
Dynamic filters based on Mach-Zehnder interferometers, and more general arrayed waveguide grating filters, have been proposed and fabricated using integrated waveguide technology. Combinations of Bragg gratings and thermo-optic phase shifters are used to realize cascaded Mach-Zehnder interferometers. These integrated waveguide dynamic spectral filters have advantages associated with fabrication. Using photolithographic wafer processing techniques, completely integrated systems have been made.
The problem with these waveguide dynamic filters, however, concerns their size, response time, and polarization anisotropy. Typically, only a few filters can be fabricated on a wafer. Further, the modulation of the thermo-optic components can be relatively slow. Although this problem can be mitigated with good design, polarization anisotropy inherent in integrated waveguides is a more pernicious problem. In effect, the operation of the filter under otherwise static conditions changes due to changes in the polarization of the input light.
Two general approaches exist for addressing polarization anisotropy. A polarization homogenizer or scrambler can be used upstream of the waveguide dynamic spectral filter. This converts an input signal having an arbitrary or random polarization into an unpolarized signal. Scramblers typically add three decibels (dB) of insertion loss, however.
A second option is to use a polarization beam splitter and two waveguide spectral filters, one for each polarization. The problem here, however, is the detrimental impact to the system size and power requirements. Moreover, the unified control of the two filters is now required.
The present invention is directed to a dynamic optical spectral filter. Different from previous such filters, the present invention is directed to a microelectromechanical system (MEMS) implementation. Such implementations can be small, operate high speed, and be made isotropic with respect to polarization.
In general, according to one aspect, the invention features a dynamic optical spectral filter. It comprises a frame. An array of mirrors is provided on a first portion of the frame, along with a second array of adjustment mirrors on a second portion of the frame. An array of variable beam splitters is provided on a middle portion of the frame, between the first array and the second array. Finally, optical delays are disposed in beam paths between the first mirror array the second mirror array. These components yield cascaded or series Mach-Zehnder interferometers that can be collectively tuned to provide an arbitrary net filter function.
According to a preferred embodiment, the first mirror array, the second mirror array, and the beam splitter array form successive stages. These stages are preferably organized in a cascade or serial configuration. The optical delay in each of these stages is different to thereby yield different spectral periods (free spectral ranges) for the interferometers of each stage.
According to a specific embodiment, the optical delays for each of the stages are integer multiples of the smallest delay. This provides for Fourier series-like behavior that helps in obtaining the desired filter transmission profiles using control algorithms.
Further, according to the preferred embodiment, the adjustable mirrors are separate deflectable mirrors. Preferably, these are implemented as out-of-plane deflecting mirrors, which are preferably deflected using electrostatic forces or voltages. The variable beam splitter array is preferably implemented as short-cavity tunable Fabry-Perot cavities. Low finesse cavities with a very large free spectral range can be used to yield a relatively uniform reflectivity across the wavelength band of interest.
In general, according to another aspect, the invention features a dynamic optical spectral filter comprising cascaded Mach-Zehnder interferometers. Each of these interferometers includes a beam splitter comprising a short-cavity tunable Fabry-Perot cavity, a first mirror, and a second adjustable mirror. Typically, at least some of these interferometers include a discrete optical delay on one of the interferometer arms.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.